This invention relates to the field of measurements while drilling, and more specifically to planning and analysis of the drilling process.
Drag and torque loss affect the drilling of all hydrocarbon wells, and are especially problematic in deviated wells. Drag manifests itself as an extra load over and above the rotating string weight when tripping out of the hole. Torsional loss from the rotating drill string while drilling causes the power available for rock destruction to be considerably lower than that applied at the rotary table. Problems of drag and torque loss normally occur together and can be particularly marked in long reach wells.
There are a variety of sources of drag and torque loss including differential sticking, keyseating, hole instabilities, poor hole cleaning, and the frictional interaction associated with side forces along the drill string. The side forces profile is essentially determined by well geometry, and can be broadly divided into the effects of poor hole conditions or inappropriate mud weight, and effects of the well path itself.
U.S. Pat. No. 4,549,432 to Soeiinah (assigned to Mobil Oil Corporation) discloses a method of detecting some of these problems in the drilling of a well from uphole measurements of hook load and free rotating torque. But experience has shown that noticeable differences occur between the torque and weight applied at the surface and that effectively applied at the bit, especially in areas of potential drilling problems. Likewise, the hookload values and the weight of the drill string in mud usually differ. Thus, the technique of the Soeiinah patent has serious inherent limitations.
The 1983 paper, "Torque and Drag in Directional Wells--Prediction and Measurement," by C. A. Johancsik, D. B. Friesen, and Rapier Dawson (IADC/SPE 1983 Drilling Conference, Paper No. 11380), proposed a computer model of drill string torque and drag, but like the Soeiinah method, this model suffers from failure to analyze downhole torque and weight parameters.
Because the available techniques lack a way of investigating and analyzing downhole torque and weight on bit, which may differ significantly from the corresponding surface measurements of torque and hookload, there remains a gap between planned optimization of a drilling program and its implementation. Thus, a need has arisen for a new technique by which torque and weight transfer along the drill string can be analyzed, both in real-time for diagnosis of drilling problems and in advance for planning.